Signal level control system



NOV. 25, 1947. I w, s m z s 1 2,431,511

SIGNAL LEVEL CONTROL SYSTEM Filed D90. 1, 1945 UTILIZATION CIRCUITROBERT w. SANDERS ATTORNEY Patented Nov. 25, 1947 2,43l,5ll

SIGNAL LEVEL CONTROL SYSTEM Robert W. Sanders, Fort Wayne, Ind,assignor, by mesne assignments, to Farnsworth Research Corporation, acorporation of Indiana Application December 1, 1945, Serial No. 632,154

9 Claims. (01. 179-171) This invention relates to signal amplifiers andparticularly to the facilities for efiecting a signal level control ofsuch amplifiers.

In many cases the intelligence signals which it is desired to amplify,vary appreciably in their level or average peak-topeak value by reasonof corresponding variations in the intelligence which is to be conveyedby such signals. One in stance of this nature is in a television systemwherein the level of the video signals may vary appreciably inaccordance with variations of the average illumination of the televisionsubject. For known reasons of amplifier design and transmissionrequlrements, it is desirable to maintain the level of such signalsreasonably constant irrespective of corresponding variations of thesubject intelligence and at the same time to avoid discrimination of thesignals at one end. or the other of the range. In other words, it isdesirable to effect a substantially linear amplification of the signals.

In a television system, the average peak-topeak value of the videosignals representative of a given television subject varies in directproportion to the general illumination of the subject. The signalgenerating device usually is of a character which responds to lightreflected from the subject. Regardless of the intensity of the subjectillumination, totally black portions of the subject will reflect nolight. Consequently, the video signal generated to represent a, blackelemental area of the subject will invariably have the same value.However, lighter portions of the subject will reflect light varying inintensity in proportion to the intensity of the general subjectillumination. For example, under relatively low intensity subjectillumination'su'ch as encountered out-oidoors on a cloudy day, the'lightreflected from a white portion of the subject, and consequently therepresentative video signal, will be of relatively small magnitude,differing in a relatively small degree from a black representative videosignal. In other words, the peak-to-peak value of such video signals issaid to be low. However, the sames'ubject in bright sunlight will causethe reflection from a white portion thereof of relatively high intensitylight, as a result of which representative video signal of. relativelylarge magnitude will be generated. This signal will differ in arelatively large degree from the black representative signal and thepeak-to-peak value of such video signals isfsaid to be high.

If video signals such as the two extreme signals describedwere to beampl fied by means of apparatus which is "not provided with anautomatic,

gain control, a corresponding disparity between the peak-to-peak valuesof the signals would 'be' produced in the amplified signals. It is,however, desirable that the amplified signals of the same televisionsubject, for example, under varying conditions of general illumination,have'substantially the same peak-to-peak values. This end may beachieved by providing the amplifier with automatic gain facilitieswhereby the gain or amplification factor is suitably reduced whensignals having a relatively high peak-to-peak value are impressedthereon.

In one type of television transmission system, the initially generatedvideo signals representative of a subject are in the form of freeelectrons. It is customary to amplify this electron signal current bymeans of a multistagestat'ic electron multiplier. A multistagemultiplier is essentially a device which is operated by impressingpositive potentials of increasing values upon successive ones of thesecondary emissive multiplier electrodes or dynodes as they aresometimes called. It ordinarily is convenient to provide the electronaccelerating potentials for the dynodes from taps on a voltage dividerto which is connected one or more sources of relatively highunidirectional voltage. The multiplied stream of electrons whichultimately is collected by a collector electrode is caused to traverse aload impedance device for the purpose of developing a signal voltagerepresentative of a television subject.

When an electron multiplier is operated in some of the arrangements ofthe prior art, there is effected a signal level control of the outputcircuit voltage by means of the values selected for the voltage dividercomponents. When the voltage divider consists entirely of relativelyhigh impedance components, the overall multiplication ratio of thedevice is less at high electron current levels than at low levels. It isseen that, while in effect a level control of the intelligence signalsmay be efiected by such means, the signals developed in the outputcircuit will be distorted by reason of the fact that the high currentsignals will not be multiplied as much as the low signal currents.

It is the object of this invention, therefore, to provide an improvedsignal amplifier incorporating means whereby a levelcontrol of theamplified signals may be effected and at the same time a linearamplifier response may be maintained.

Another object of the invention is to provide a novel coupling betweenan electron multiplier and a vacuum tube amplifier whereby to linearizethe output of the amplifier and at the same time to maintain asubstantially constant signal level.

In accordance with this invention, there is provided a multistageelectron multiplier having at least one but preferably a plurality ofdynodes and an electron collector electrode. Electron acceleratingpotentials are impressed upon the dynodes by means including one or moreimpedance devices. In a preferred form of the invention, the dynodes areconnected to diiferent points on a voltage divider which is furnishedwith energy from a source of unidirectional current. In order tolinearize the output signal voltage of the multiplier, the voltagedivider components connected to the higher stage dynodes are ofrelatively low impedance as compared to the relatively high impedance ofthe divider components connected to the lower stage dynodes. There alsois provided a vacuum tube amplifier having an input circuit coupled tothe output circuit of the multiplier which is derived from the collectorelectrode. The input circuit of the amplifier tube also is connected toone of the voltage divider impedance devices which is coupled to themultiplier in such a manner to be traversed by the electron spacecurrent flow in the multiplier. A biasing voltage for the amplifier thusis developed by the divider impedance device. The output circuit of theamplifier tube is connected to a utilization circuit. The biasingvoltage which is impressed upon the input circuit of the tube fluctuatesin accordance with variations of the electron space current and therebyfunctions to automatically control the gain of the amplifier in asuitable manner to effect the desired Signal level control thereof,

For a better understanding of the invention together with other andfurther objects thereof, reference is made to the following description,taken in connection with the accompanying drawing, and its scope will bepointed out in the appended claims.

The single figure of the accompanying drawing is a schematic circuitdiagram of an illustrated embodiment of the invention.

Having reference now to the drawing, the source of signal effects isillustrated as an electron multiplier which may be assumed to beassociated with an image analyzing tube em bodied in a televisionsystem. Such a tube functions to convert an optical image of atelevision subject systematically into electronic signal effectsrepresentative of different elemental areas of the subject. Only a fewof the stages of such an electron multiplier are shown for the reasonthat the invention relates principally to the higher stages of such adevice and to the coupling facilities between a multiplier of thischaracter and a vacuum tube amplifier. The multiplier may comprise anynumber of stages required to meet the needs of the television system inwhich it is incorporated. The multiplier is mounted within an evacuatedenvelope 1 and comprises a series arrangement of a plurality ofsecondary electron emissive electrodes or dynodes, such as the dynodes 2to 8 inclusive. These dynodes are illustrated as having conventionalbox-like forms but it is to be understood that they may have any otherconfiguration desired without departing from the scope of the presentinvention. Adjacent the last stage dynode 8, there is disposed theelectron collecting electrode 8 which may have the form of asubstantially flat plate, as illustrated,

Each of the multiplier dynodes is maintained at a more positivepotential than its l q$59- These potentials or electron acceleratingpotentials are derived from a voltage divider H which comprises a seriesconnection of a plurality of impedance devices such as resistors 12 toIt inclusive. Energy is supplied to the voltage divider from anyconvenient source of unidirectiona1 current. In the present instance,this source is illustrated as two batteries 19 and 2| connected inseries. The positive terminal of the battery 2 l, which is grounded, isconnected through the resistor 2! to the voltage divider resistor I!which is connected to the last stage multiplier dynode ii. In thepresent instance, the divider resistors 55, I6 and H, which areassociated with the higher stage dynodes 5, 6 and"! respectively, are ofrelatively low impedance, of the order of 30,000 ohms. The dividerresistors I2, I3 and M, which are associated with the lower stagedynodes 2, 3 and 4 respectively, are of relatively high impedance, ofthe order of 200,000 ohms. The divider resistor is, associated with thelast stage dynode S and the collector 9, also is of relatively highimpedance and is shunted by a condenser 22 to bypass the high frequencycomponent of the video signals for a purpose to be described. Thejunction point between the batteries l9 and 2! is connected to thejunction point between the high impedance resistor I4 and the lowimpedance resistor IS. The battery I9, therefore, may be of lower outputcapacity than the battery 2!.

The collector electrode 9 is connected to ground through an output loadimpedance device, such as a resistor 23, and a source of negativebiasing voltage, such as a battery 24. The collector electrode 9 also iscoupled to the control grid of a vacuum tube 25. The cathode of thistube is connected to a sliding contact 25 which is en gaged with thevoltage divider resistor it. The tube cathode also is connected toground through a high frequency video signal by-pass condenser 27. Theanode of the tube 25 is coupled to one input terminal of a utilizationcircuit 28, the other input terminal of which is connected to ground.

Referring now to the operation of the apparatus embodying thisinvention, assume that it is to be used in a television signaltransmitting system. In this case the electron multiplier may beassociated with an image analyzing tube of the dissector type. In anapparatus of this character an optical image of the television subjectis projected onto a photoelectric cathode at one end of the tube,whereby to efiect an electron emission from various parts of the cathodecorresponding in magnitude to the intensity of the subject-reflectedlight projected onto the cathode. The emitted electrons are formed intoan orderly arrangement of a plurality of elemental dynodes are mostconveniently derived from taps on a voltage divider such as the dividerl I. These accelerating potentials are produced in the dividercomponents by the flow of current therethrough. In the absence of anyelectrons in the multiplier, the current which flows through the voltagedivider is solely the result of the connection of the batteries I9 and2| thereto, and its magnitude is determined only by the impedance of thevoltage divider components and the voltage of the power supply battery.However, when an electron space current flows between the series ofmultiplier dynodes, the path between any two adjacent dynodes iseffectively connected in shunt with one of the voltage dividercomponents. In a multistage electron multiplier such as that described,the current flow through the voltage divider is exceedingly complex byreason of the multiplicity of such path around the components of thedivider.

For an understanding of the present invention, however, it is notnecessary to deal specifically with the details of the currentdistribution in the voltage divider. It is well known in the art that,when using in connection with a multistage electron multiplier a voltagedivider consisting entirely of relatively high impedance components, thevoltages developed in the individual components will vary withvariations in the electron space current flow in the multiplier. With nospace current flowing in the multiplier and using a divider made up of anumber of high impedance components which are all of the same value, thevoltages developed are equal for all components. In other words, thevoltage distribution in the electron multiplier is linear. In the casewhere an appreciable electron space current flow is produced through themultiplier, the voltages developed across the divider componentsassociated with the low stage dynodes, wherein the electron currentdensity is relatively low, are greater than the normal voltagesdeveloped therein when there is no electron current flow in themultiplier. Conversely, with a substantial electron space current flowin the multiplier, the voltages developed across the divider componentsassociated with the higher stage dynodes, wherein the density of theelectron current is relatively great, are less than the voltagesdeveloped therein when there is no electron space current flow. Thetotal voltage drop across the entire divider, of course, is the same inboth cases, being equal to the total voltage of the power supply such asthe batteries [9 and 2!.

So long as the electron space current flow in the multiplier is not toogreat, the overall multiplication factor of a multiplier energized inaccordance with prior art practice is not changed appreciably as theresult of the described variations in the individual electronaccelerating potentials applied to the respective dynodes. However, whenthe electron space current flow in the multiplier rises to a relativelyhigh value, the multiplication factor of the multiplier decreases. Thedevice no longer functions in a linear manner but, instead,discriminates against the elec-, tron current flow in the higher regionoithe range of variation with the result that the signal efiectsproduced are distorted at the high end of the signal range.

It is considered that the reason for the decrease in the acceleratingvoltages developed across the voltage divider components associated withthe higher stage multiplier dynodes is that the electron current flowingin shunt with the divider components is too great in proportion to thenormal current flow through the associated ivider components.

However, by making the divider components such as l5, l6 and 11associated with some or all of the higher stage dynodes of a much lowerimpedance than the components such as the resistors l2, l3 and M, theratio of the shunting electron current to the normal current through theassociated divider component is smaller. The result is that theaccelerating voltage developed in a resistor such as l5, 16 or i! ismaintained sufficiently high to effect the necessary electronmultiplication in the higher stages to maintain the overallmultiplication factor of the device substantially constant and therebymaintain a substantial linearity of response by the device.

Consequently, in accordance with one feature of the present invention,the voltage divider resistors l5, l6 and I! are of much lower impedancethan the resistors I2, [3 and M as described. Then, when an electronmultiplier such as that described is used in conjunction with animageanalyzing tube of a television system, the signal voltagesdeveloped in the output resistor 23 bear a substantially linearrelationship to the light values of the television subject irrespectiveof the intensity of the general illumination of the subject. As aconsequence, however, the signal voltages developed in the outputresistor 23 will have an average peak-to-peak value which, underconditions of bright illumination of the television subject, are greaterthan it is convenient to amplify and transmit to a receiving point.

Therefore, in order to compensate for such a variation in the averagepeak-to-peak value of the generated signals, there is provided, inaccordance with another feature of this invention, a facility forautomatically varying the gain or amlification factor of an amplifierinversely to variations in the average peakto-peali value of thesignals. This facility consists of a means for impressing upon thecathode of the vacuum tube 25 a biasing voltage which varies in asuitable manner in response to variations of the average peak-to-peakvalue of the signals. The voltage divider resistor i8, which isby-passed by the condenser 22 for the high-frequency instantaneoussignal values representative of elemental areas of the televisionsubject, develops, therefore, a voltage which is representative of theaverage peak-to-peak value of the signals. The voltage developed in theresistor i8 varies inversely to the fluctuation of the electron currentin the multiplier. As previously explained, this results from the factthat the resistor has a relatively high impedance and is associated withone of the higher stage dynodes. As the average light of the televisionsubject increases, the voltage developed across the resistor itdecreases. A decrease in this developed voltage makes the slider 25, andtherefore the cathode of the tube 25, more positive, thereby biasing thetube in such a manner to decrease its gain. The video signals developedin the resistor 23, even though they have an average peak-to-peak valuewhich is greater than normal, will be amplified to a lesser degree bythetube 25 as a result of the cathode biasing so that, in the outputcircuit of the amplifier, the average peak-to-peak value of the signalswill remain substantially constant irrespective of variations of theaverage subject illumination.

Of course as the average peal -to-peak value of the signals developed inthe resistor 23 de creases, the low-frequency voltage developed in thedivider resistor l8 increases. The voltage impressed upon the cathode ofthe tube 25 by the slider 26 becomes more negative, thereby increasingthe gain of the amplifier tube 25 suitably to maintain the signal levelin the output circuit thereof at the desired substantially constantvalue.

The biasing voltage which is impressed upon the cathode of the tube 25in the manner described for effecting an automatic gain control of theamplifier is caused to vary at the relatively slow rate of the lowfrequency component of the video signals. In other Words, this biasingvoltage represents the average illumination of the television subject.The biasing voltage is developed in the resistor 58 to represent theaverage subject illumination by reason of the connection of the by-passcondenser 22 in shunt with this resistor. By this means the relativelyhigh frequency component of the video signals representing theinstantaneous light values of the subject are caused to traverse theby-pass condenser which has a suitable value for achieving this result.Consequently, only the relatively low frequency component of the videosignals representing the average illumination of the subject traversesthe divider resistor l8 for the development of the biasing voltage.

By reason of the illustrated connection of the vacuum tube 25 to aportion of the divider resistor 18, the space current in the tube alsotraverses a part of this divider resistor. The instantaneousfluctuations of the space current in response to the relatively highfrequency component of the video signals impressed upon the control gridare by-passed around that portion of the resistor I8 traversed by thespace current by means of the condenser 21. This condenser also isrequired 7 to have a value suitable to provide a relatively lowimpedance for the high frequency component of the video signals.

The most satisfactory values of the by-pass condensers 22 and 27 aresuch that, in conjunction with the respective portions of the dividerresistor associated therewith, the time constant of the circuits isrelatively great. For optimum results this time constant should be ofthe order of the time required to scan several fields of the televisionpicture.

While the instant invention has been described in a preferred embodimentwherein the cathode of the vacuum tube is biased to eifect the desiredgain control of the amplifier, it is considered to be within the scopeof the invention to employ other facilities incorporating the underlyingprinciples of the invention to effect the gain control of the amplifierin the manner desired. For

example, it is known that the current variations in some of the othervoltage divider components, particularly those associated with some ofthe lower stage dynodes are in a sense opposite the current variationsin the resistor I 5. In other words the current in some of these otherdivider resistors increases with an increase in the average lightreflected from the subject. The voltage developed in such a resistoralso may be employed to effect the desired gain control of theamplifier. However, in such a case, the voltage should be impressed uponthe control grid of the tube 25 in any manner within the knowledge ofthose skilled in the art.

While there has been described what, at present, is considered thepreferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it, therefore, isaimed in the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

What is claimed is:

1. An amplifier for television video signal effects having a relativelylow frequency component representative of average subject illuminationcomprising, an electron multiplier having a dynode and an electroncollector electrode, an impedance device connected to said dynode, anoutput circuit for said multiplier including a load impedance devicecoupled to said collector electrode, a vacuum tube having an inputcircuit and an output circuit, means for coupling said input circuit tosaid multiplier output circuit, means for coupling said input circuit toone of said impedance devices for varying the amplification factor ofsaid tube oppositely to variations of said low frequency signalcomponent, and a utilization circuit coupled to the output circuit ofsaid tube.

2. An amplifier for television video signal effects having a relativelylow frequency component representative of average subject illuminationcomprising, an electron multiplier having a dynode and an electroncollector electrode, an impedance device connected to said dynode fordeveloping a voltage depending upon the magnitude of the multiplierspace currents, an output circuit for said multiplier including a loadimpedance device coupled to said collector electrode, a vacuum tubehaving an input circuit and an output circuit, means for coupling saidinput circuit to said multiplier output circuit, means for coupling saidinput circuit to the impedance device connected to said dynode forbiasing said tube oppositely to variations of said low frequency signalcomponent, and a utilization circuit coupled to said tube outputcircuit.

3. An amplifier for television video signal effects having a relativelylow frequency component representative of average subject illuminationcomprising, a multistage electron multiplier having a plurality ofdynodes and an electron collector electrode, a plurality of impedancedevices connected respectively to said dynodes and traversed by electronmultiplier currents, an output circuit for said multiplier including aload impedance device coupled to said collector electrode, a vacuum tubehaving an input circuit and an output circuit, means for coupling saidinput circuit to said multiplier output circuit, means for coupling saidinput circuit to one of said impedance devices for varying theamplification factor of said tube oppositely to variations of said lowfrequency signal component, and a utilization circuit coupled to theoutput circuit of said tube.

4. An amplifier for television video signal efiects having a relativelylow frequency component representative of average subject illuminationcomprising, a multistage electron multiplier having a plurality ofdynodes and an electron collector electrode, a source of acceleratingpotentials for said dynodes including a voltage divider comprising aplurality of impedance devices, an output circuit for said multiplierincluding a load impedance device coupled to said collector electrode, avacuum tube having an input circuit and an output circuit, means forcoupling said input circuit to said multiplier output circuit, means forcoupling said input circuit to one of said impedance devices for varyingthe amplification factor of said tube oppositely to 9 variations of saidlow frequency signal component, and a utilization circuit coupled to theoutput circuit of said tube.

5. An amplifier for television video signal effects having a relativelylow frequency component representative of average subject illuminationcomprising, a multistage electron multiplier having a plurality ofserially arranged dynodes and an electron collector electrode, a sourceof accelerating potentials for said dynodes including a voltage dividercomprising a plurality of impedance devices, an output circuit for saidmultiplier including a load impedance device coupled to said collectorelectrode, a vacuum tube having an input circuit and an output circuit,a coupling between said input circuit and said multiplier outputcircuit, a coupling between said input circuit and one of said voltagedivider impedance devices for biasing said tube oppositely to variationsof said low frequency signal component, and a utilization circuitcoupled to said tube output circuit.

6. An amplifier for television video signal effects comprising, amultistage electron multiplier having a plurality of serially arrangeddynodes and an electron collector electrode, a source of acceleratingpotentials for said dynodes including a voltage divider consisting of aseries connection of a plurality of impedance devices, the impedancedevices associated with the lower stage dynodes being respectively ofrelatively high value and the impedance devices associated with some ofthe higher stage dynodes being of relatively low value, an outputcircuit for said multiplier coupled to said colector electrode, a vacuumtube having an anode, a cathode and a control grid, a coupling betweensaid control grid and said output circuit, a coupling between saidcathode and the impedance device associated with one of said higherstage dynodes, a condenser connected between said cathode and a point onsaid cathode coupled impedance device to provide a low impedance by-passcircuit for relatively high frequency signal components of said signaleffects, and a utilization circuit coupled to said anode.

7. An amplifier for television video signal efiects comprising, amultistage electron multiplier having a plurality of serially arrangeddynodes and an electron collector electrode. a source of acceleratingpotentials for said dynodes including a voltage divider consisting of aseries connection of a plurality of impedance devices, the impedancedevices associated with the lower stage dynodes being respectively ofrelatively high value and the impedance devices associated with some ofthe higher stage dynodes being of relatively low value, an outputcircuit for said multiplier connected to said colector electrode, avacuum tube having an anode, a cathode and a control grid, a couplingbetween said control 10 grid and said output circuit, a coupling betweensaid cathode and the impedance device associated with the last stagedynode, a condenser connected between said cathode and the positiveterminal of said voltage divider to provide a low impedance by-passcircuit for relatively high frequency signal components of said signaleffects, and a utilization circuit coupled to said anode.

8. An amplifier for television video signal cfiects comprising, amultistage electron multiplier having a plurality of serially arrangeddynodes and an electron collector electrode, a source of acceleratingpotentials for said dynodes including a voltage divider consisting of aseries connection of a plurality of resistors, the resistors associatedwith the last stage dynode and with the lower stage dynodes beingrespectively of relatively high impedance and the resistors associatedwith the intermediate dynodes being of relatively low impedance, a loadimpedance device connected to said collector electrode, a vacuum tubehaving an anode, a cathode and a control grid, a coupling between saidcontrol grid and said load impedance device, a coupling between saidcathode and the voltage divider resistor associated with the said laststage dynode, a condenser connected in shunt with said last stage dynodedivider resistor to provide a low impedance by-pass circuit forrelatively high frequency components of said signal efiects, and autilization circuit coupled to said anode.

9. An amplifier for television video signal effects comprising, amultistage electron multiplier having a plurality of serially arrangeddynodes and an electron collector electrode, a source of acceleratingpotentials for said dynodes including a voltage divider consisting of aseries connection of a plurality of resistors, the resistors associatedwith the last stage dynode and with the lower stage dynodes beingrespectively of relatively high impedance and the resistors associatedwith the intermediate dynodes being of relatively low impedance, a loadresistor connected between said collector electrode and the positiveterminal of said voltage divider, a thermionic amplifier including avacuum tube having an anode, a cathode and a control grid, a couplingbetween said control grid and said load resistor, a coupling betweensaid cathode and the voltage divider resistor associated with said laststage dynode, a condenser connected in shunt with said last stage dynodedivider resistor to provide a low impedance by-pass circuit forrelatively high frequency components of said signal effects, a condenserconnected between said cathode and the positive terminal of said voltagedivider to provide a low impedance by-pass circuit for said relativelyhigh frequency signal components, and a utilization circuit coupled tosaid anode.

ROBERT W. SANDERS.

